Integrated process for coking spent base sulfite pulping liquors

ABSTRACT

In the coking of spent ammonia and low pH sodium base sulfite wood pulping liquors in the liquid phase under pressure, gel-type coke formation is avoided by rapid heating to the coking temperature. Such rapid heating can be accomplished by the use of a metal bath having high heat transfer capacity, by using high pressure-high temperature steam injection or by means of a fluidized bed combustion system. The heating rate used is one of about 110* to about 150*F. per minute in the temperature interval of about 350* to about 550*F.

iltte States atent 1191 1111 3,864,205 Franz et al. 1 Feb. 4, 1975 1 INTEGRATED PROCESS FOR COKING 2,947,656 8/1960 Bailey 162/36 x SPENT BASE SULFITE PULPING LIQUORS g 1 5;

, ra acs 6 a 1 Inventors: William Franz, Gardiner; Howard 3,464,917 9/1969 POI'ICOLIS 210/71 x V. Hess, Glenham; Edward L. Cole, 3,558,426 1/1971 Hess et al. 162/30 Fishkill; Kenneth E. Neisser, 3,591,449 7/1971 Hess et a1. 162/30 Poughkeepsie all of N Y' 3,649,534 3/1972 Schotte 210/63 [73] Assignee: Texaco Inc., New York, N.Y. S

Primary Examiner- Leon Bashore [22] Flled: 1973 Assistant Examiner-Alfred DAndrea, Jr. [21] Appl. No.: 391,908 Attorney, Agent, or Firm-T. H. Whaley; C. G. Ries Related U.S. Application Data [63] Continuation-in-part of Ser. Nos. 149,673, June 3,

1971, abandoned, and Ser. No. 149,577, June 3, [57] ABSTRACT 1971, abandoned. In the coking of spent ammonia and low pH sodium base sulfite wood pulping liquors in the liquid phase under pressure, gel-type coke formation is avoided by [52] U.S. Cl 162/31, 162/36,412632//1467l, rapid heating to the coking temperature. Such rapid I Cl D21 11/14 heating can be accomplished by the use of a metal i 63 21. bath having high heat transfer capacity, by using high [5 Fred 620516226 36 i7 im/ 20 pressure-high temperature steam injection or by 423/461 means of a fluidized bed combustion system. The heating rate used is one of about 110 to about 150F. per minute in the temperature interval of about 350 [56] References Cited to about 6 UNITED STATES PATENTS 2,739,039 3/1956 Phelps 162/36 X 8 Claims, 3 Drawing Figures E RECOVERED NH MAKE-UP u RECONSTITUT'ED PULPING LIQUOR a LEAN SOLVENT S0 RECOVERY E RICH SOLVENT T0 s0 STRIPPING RELATED U.S. APPLICATION DATA dry coke suitable for firing as plant fuel.

SUMMARY OF THE INVENTION The invention resides in an improvement in a process This application is a continuation-in-part of coas- 5 for coking liquors of the character described by heat signed patent application Ser. No. l49,673 and treating such liquors in a coking zone at a temperature l49,577 both filed on June 3. 197], now abandoned, range of about 400 to about 700F under pressure at said applications both being continuations-in-part of the selected temperature such that coking takes place commonly owned US Pat. No. 3,595,742 of July 27, in the liquid phase, which improvement consists in rap- I97 l. idly heating such liquids at a rate of about I 10 to about 150F. per minute in the temperature interval of about BACKGROND OF THE INVENTION 350550F. Such rapid heating can be achieved by the 1. Field of the Invention use ofa metal bath, by high pressure-high temperature The present invention relates to an improved process steam injection or by means ofa fluidized bed combusfor the liquid phase coking of spent ammonia and low i system P Sodium base Sulfile Wood pulping q More P ln a further variation of the invention combustion in y this invention Solves the Problem of gel yp the fluidized bed is carried out in the presence ofa limcoke formation encountered in the liquid phase coking i d amount f Oxygen h that h primary product of Such liquors and facilitates Coke Separation of combustion of the carbon in the coke will be CO.

Dt-scrlptlon ofthe Prior Art Through the water gas shift reaction this CO is con- A recently dcvelopcd ProcesS of liquid Phase coking verted to H and CO Subsequent removal and recovrepresents a great simplification over the existing ery f CO leaves N gas mixture i m f method of handling P p p g liquors Since it charging to an ammonia converter. The converter used quires only a heater and a hold tank- Chemi al I'EC for this purpose is operated at relatively low pressure ery, too, is greatly simplified in that the major portion since low per pass conversions are acceptable here. of the chemicals is recovered in solution in the coker The dilute ammonia produced is adsorbed in the coker li id effluent. Even heat recovery from the spent liliquid effluent to replace lost ammonia and to reconstiquor is greatly simplified. The fuel to be burned is a tute it to pulping composition. In this embodiment of granular coke-like material, rather than the viscous the invention all or part of the CO separated can be reconcentrated spent pulping liquor. cycled to the fluid bed coke burner. The recycled C0 The liquid phase coking process is itself of great adaids in temperature control, by its endothermic reducvantage in handling the spent ammonia base sulfite lition to CO by the carbon. By controlling the CO conquors. In conventional recovery systems where the centration in the combustion air to the fluidized bed spent liquor is first concentrated, then burned, essencombustor, the CO concentration in the flue gas can be tially all of the ammonia is lost. When these spent Iicontrolled and hence the H concentration in the quors are handled in the liquid phase coking process, N -H gas stream fed to the ammonia converter. about 70807c ofthe ammonia in the spent liquor is re- As used herein, low pH sodium base sulfite liquors" covered in solution in the liquid effluent from the include liquors of the character described which have coker. The remainder of the ammonia appears as the a pH in the range of l to 3. nitrogen content ofthe coke and would represent a loss of ammonia to the overall pulping-recovery operation. DESCRIPTION OF THE DRAWINGS While liquid phase coking represents a big improve- FIG. I is a diagrammatic illustration of an embodiment over conventional recovery processes in terms of ment ofthe invention wherein rapid heating is achieved ammonia recovery, there is still a considerable loss to by means of a liquid metal. the coke. Make-up ammonia or ammonium salts are FIG. 2 is a flow diagram showing another embodinecessary to replace this loss and can be an expensive ment of the invention wherein a rapid heating rate is item due to location of many pulping plants remote obtained by steam injection. from any sources of ammonia. FIG. 3 is a flow diagram showing a further embodi- A further difficulty has been encountered in the ap- 5O ment of the invention employing a fluidized bed complication of liquid phase coking to spent ammonia sulbustion system. fite pulping liquors. lf over the temperature range from One of the important features of the invention that about 350F to the coker operating temperature of the rate of heating during the coking operation is an im- 550-650F. the rate of heating is too slow, the coke portant variable in determining the properties of the will be formed as a gel of high water content. This gel coke produced is demonstrated by the data in Table structure is separated only with difficulty to obtain a I belowr TABLE I Soaking Filtrate Wet Filter Dry Filter Operating Heating Time at Filtration Yield Coke Yield Coke Yield TemplF Time-Min Temp-Min Time Sec Wt.% Wt.% Wt.%

550 3.5" 20.0 l9 63.2 34.9 7.5 550 3.5" 120 b 65.3 32.3 7.3 550 I55 2100 I92 77.6 8.6 550 I00 60 2100 32.4 65.0" 8.2

") Heated by immersing sample bomb in liquid metal bath maintained at specified operating temperature.

") Filtration rate equivalent to 980 Ib/hr-ft.

"J Heated by immersing sample bomb in liquid metal bath at 380F. then raising temperature of bath containing the sample bomb to operating temperature over specified time interval.

") Filter coke in the form ofa gel of high water content.

') Heated by electric heating mantle over the specified time interval.

The data in Table I were obtained thus:

Samples of spent ammonia base sulfite cooking liquor (about 60 g samples) were placed in steel tubes and heated to coking conditions by immersion in a molten metal bath. The rate of heating was controlled by varying the bath temperature. It had been empirically determined that immersion of the filled sample tube in the metal bath maintained at coking temperature (500-550F) required about three minutes to bring the sample tube and contents into thermal equilibrium with the molten metal bath. This heating time represents one extreme. At the other extreme, the steel sample container was heated over a period of 2.5 hours in an electric heating mantle. As shown in Table 1, the rapid heating resulted in a readily separated coke from which a clear filtrate could be obtained at a high filtration rate. In contrast, the sample heated slowly by means of the electric mantle coked to form a gel structure from which the coke was separated by filtration only with difficulty. The experimentally observed filter time indicates that filtration of the coked product obtained at a slow heating rate would be impractical in a commercial operation. The foregoing clearly indicates the criticality of the heating rate in successfully coking a spent ammonia base sulfite pulping liquor. Available evidence suggests that this critically high heating rate is required over only a relatively narrow temperature range, rather than over the entire range from ambient to coking temperature. Since ammonia base sulfite pulping is conventionally carried out at cooking temperatures of 285-340F with no difficulties encountered due to gel formation in the spent liquor or coking during the digestion process, it follows that the high heating rate is required only at temperatures above the normal cooking temperatures. To verify this point experimentally, another set of data was obtained. In this instance, the molten metal bath was heated to 380F. At this temperature the steel bomb containing 60 g. of spent liquor was immersed in the bath and the temperature raised to the 550F coking temperature over a period of more than an hour and a half. As the last set of data given in Table I shows. this operation again produced a coke in the gel form. As in the previous case, separation of the coke to obtain a clear filtrate for chemical recovery and reconstitution was not practical. Thus in applying liquid phase coking to spent ammonia base sulfite wood pulping liquors, the purpose of chemical recovery and recycle to the pulping operation can be accomplished by restricting the high heating rate to temperatures above about 350400F. N

Further experiments showed that the high rate of heating to be maintained over the temperature range of about 350F to about 550F must be about 110 to l50F/minute and preferably 135 to 140F/minute.

In the embodiment of the invention shown in FIG. I, spent ammonia base sulfite pulping liquor is raised to coker operating pressure of about 1000 psi by charge pump 11 and preheated to about 300-350F by exchange against coker effluent in heat exchanger 12. The preheated liquor is then raised to coking temperature of about 550F by indirect heat exchange with a The heated spent sulfite liquor then passes from exchanger 14 to the coker hold drum 16, where sufficient residence time is provided to complete the coking reaction (0.5-60 minutes) and to allow the coke formed in the reaction to settle to the bottom of the drum. Clear effluent is removed from the coker hold drum through line 17 and passes through preheat exchanger 12 where it is cooled by heat exchange against incoming spent sulflte liquor. The clear coker effluent may then be further cooled in exchanger 18 and depressured through throttling control valve 19. Coker off gas is separated in gas-liquid separator 20 and passes through line 21 to the tired heater where the sulfur compounds contained in the off gas are burned to S0 The liquid coker effluent passes from separator 20 through line 22 to absorber 23. Here recovered S0 and makeup NH,, are dissolved in the liquid to reconstitute it to cooking composition for recycle to the pulping digesters through line 24.

Settled coke passes from the coker hold drum 16 through line 25 to the coke dewatering device 26. Dewatering device 26 may be a liquid cyclone, rotary drum filter or other similar type of equipment. Liquid separated from the coke passes through line 27 to join the main stream of coker liquid effluent at point 28. Dewatered coke from 26 passes to coke dryer 29 where residual water is evaporated by contact with hot flue gas from fired heater 15. Dried coke is removed from the bottom of drier 29 and transferred through line 30 to fired heater 15 where it is burned to supply process heat.

Flue gas leaves drier 29 through line 31 and passes to absorber 32. Here S0 generated by combustion of the sulfur-containing process coke as well as sulfur containing coker off gas is recovered by absorption from the flue gas. Conventional acid gas scrubbers may be used for S0 recovery. A pollutant free flue gas exists from absorber 32 and is vented to the atmosphere through line 33. A separate stripping column (not shown) regenerates S0 from the rich absorber solvent. The S0 is then added to the liquid coker effluent in absorber 23 to reconstitute it to cooking liquor composition.

Another means for accomplishing a rapid heating rate is illustrated by the flow diagram of FIG. 2. Spent wood pulping liquor from the pulping digestors blow down tank at 150F is raised to coker-operating pressure by pump 39 and is exchanged against the hot coked liquor in exchanger 40 to preheat it to the lower end of the critical heating rate temperature range. Heating to the final coking temperature, taken to be 550F in the illustrative case, is accomplished by direct injection through mix valve 42 of 0.23 pounds of superheated steam (at 1100 psi, l000F) per pound of spent liquor. After allowing a residence time of 3 to 5 minutes to complete the coking reaction, the coked liquor is exchanged against the incoming feed. The heat exchanger effluent, still at the l psi operating pressure but now cooled to 400F, then passes through line 44 and a throttling valve 46 to reduce pressure from 1100 to 50 psi and into flash drum 48. This pressure drop is sufficient to flash all of the condensate introduced in the open steam heater. Saturated steam at about 280F, together with gas produced in the coking reaction, is taken overhead from flash drum 48 and condensed in the overhead cooler 50. Uncondcnsed fixed gases, containing malodorous sulfur compounds, are further reduced in pressure from the 50 psig flash drum pressure by throttle valve 52 and then burned in the heat recovery furnace 54 with other sulfur containing gases. such as the digestor relief gases from elsewhere in the plant. Not only does this preclude emission to the atmosphere of noxious sulfur compounds but burning of these gases permits recovery of the sulfur as $0 from the furnace flue gases.

The liquid phase material from flash drum 48 containing the coke in slurried suspension, is cooled by exchange against reconstituted cooking liquor in exchanger 56 and passes to a coke separation device 58. Such a device can be a liquid cyclone, which separates the coke as a fairly dense slurry underflow. The underby the use of the liquid phase coking process. eliminates the water pollution caused by dumping spent cooking liquors into streams and lakes. Incorporation of the sulfur recovery system virtually eliminates emission of atmospheric pollutants. These pollution abatement measures are accomplished in a manner which returns what would otherwise be harmful waste products to the process as useful chemicals.

Tests have amply demonstrated that a fast heat-up time is controlling over gel formation and that the rate ofquench of the coked liquor has no bearing on gel formation. This can be observed in Table 11 below (Run 144). The other runs, 132,133 and 137 all gave low filtrate yields and a gel-like wet coke. Despite the fact that Run 137 was carried out at a higher temperature (600F) than the others, it still gave a gel.

TABLE 11 Wet Filter Dry Time to Time at Filtrate Coke Coke Run Run Temp Run Temp Yield Yield Yield Run No. TempF Min Min Wt Wt Wt 72 Remarks 132 500 165 120 23.7 77.5 7.2 Product from autoclave was a gel Very 133 550 185 120 19.0 79.2 10.1 difficult to filter.

, These products were 137 600 135 120 18.2 81.0 8.25 cooled slowly in the autoclave. 144' 550 3.5 120 65.3 32.3 7.28 Product was a readily settlable coke in liquid very fast filtering.

' Carried out in hot metal bath to obtain fast heating to run temperature. Filter ratc calculated for the product from Run 144 is 980 1b/sq.'ft/hour. Product was quenched in air to ambient temperature.

flow slurry then passes to a drying tower 60 where the associated water is vaporized by countercurrent direct contact flow with hot flue gas from the heat recovery furnace 54. Dried coke is removed from the bottom of drying tower 60 and fired to the heat recovery furnace 54. A powdered fuel burner or fluid bed combustion system can be used advantageously to burn the coke. Heat of combustion is used to generate the superheated steam required for the critical spent liquor heating as well as to raise steam for general plant use.

Overflow liquid from the liquid cyclone coke separator 58 after addition of make-up ammonia or ammonium compounds through line 62 to make up for nitrogen losses to coke, is charged to a flue gas scrubber 64 where S0 is absorbed from cooled flue gas exiting from the coke drier. Scrubbed flue gas free of noxious components may then be vented through a stack through line 66. Liquor withdrawn from the bottom of the flue gas scrubber 64 is now reconstituted to cooking composition. After exchange against the flash drum liquid effluent in exchanger 56 to heat it to cooking temperature, it is recycled to the pulping digestors through line 68.

The essential feature of the embodiment of FIG. 2 is the use of superheated steam in an open heater to achieve a high heating rate over the critical gel forming temperature range of the coking process. In addition to circumventing the gel formation problem in applying liquid phase coking to spent ammonia base sulfite wood pulping liquors, the flow results in an overall wood pulping operation with minimum release of pollutants to the environment. Reconstitution of the spent cooking liquor and recycle to the digestors, made possible Another version ofthe invention is illustrated in H0. 3. As shown, spent ammonia base sulfite pulping liquor is preheated to 300350F. in exchanger 111 and then heated to 550F. coking temperature (950-1000 psig coker operating pressure) by passage through the heating coils immersed in the fluidized coke combustion bed 112. The heater effluent then passes by line 113 to the coker hold drum 114 where sufficient residence time is allowed to complete the coking reactions and to allow the coke to settle to the bottom ofthe vessel. The clear supernatant coker liquid, after exchange against cool incoming feed in exchanger 111 is partly reconstituted by addition through line 115 of SO recovered from the coke combustion gases in $0 section 116. The partly reconstituted liquor is then pumped to the ammonia system operating pressure in pump 117 and the required make-up ammonia added by adsorption from the ammonia converter off gases in adsorber 118. Fully reconstituted pulping liquor from the ammonia adsorber is then recycled to the pumping digestors through line 119.

Settled coke is withdrawn from the bottom of the coker hold drum 114 through line 120 and partly dewatered in a liquid cyclone separator 121, or similar suitable equipment. Overflow from the cyclone joins the main liquid effluent stream through line 122 while the coke slurry underflow exits through line 123 and is dried by hot combustion gases from the heater 112 in the coke drier 124. The dried coke is then burned in the fluidized bed heater 112. Combustion temperature is controlled by dilution of combustion air with tail gas through line from the ammonia adsorber 118. Additionally, recovered CO from CO section 126 can be added to control flue gas composition as well as for temperature control.

Hot combustion gases from the coke pass through line 127 to the coker drier 124, then to S removal 116. Sulfur-free gas, with added steam through line 128 then passes through a water gas shift converter 125 where CO from coke combustion as well as from CO added to combustion air is converted to H The CO; is removed in a scrubbing system 126 and a portion of the recovered CO returned to the combustion zone to adjust flue gas composition and combustion temperature. The scrubbed gas, now containing the desired H N ratio as a result of CO recycle to the fluidized bed coke combustion is compressed to ammonia converter operating pressure in compressor 128. Since the product ammonia is to be adsorbed in the 50;, containing pulping liquor, quite low concentration in the converter exit gases, and hence quite low conversion can be tolerated. Temperatures of about 750F. at an operating pressure of 1500 psig or below are used in the am monia converter 129. Adsorption of the dilute ammonia from the converter gas in line 130 in the ammonia adsorber 118 completes reconstitution of the pulping liquor for recycle to the digestor through line 119. A portion of the tail gas from the ammonia adsorber 118 may be recycled to the ammonia converter through line 13], another portion used to dilute the coke combustion air through line 145 and the remainder can be vented through line 132.

The foregoing clearly indicates the process whereby the present invention avoids the formation of gel type coke when spent wood pulping liquors are coked. The benefits afforded paper making operations and technique will readily be ascertained by those skilled in this art.

While the invention has been illustrated with physical embodiments, these are exemplary only and the invention is limited only by the subjoined claims.

What is claimed is:

1. in a process for coking spent ammonia and low pH sodium base sulfite wood pulping liquors wherein the liquors are coked in the liquid phase under pressure in a coking zone at a temperature in the range of 400 to 700F. to produce coke, gases including sulfur dioxide, and an effluent, the improvement consisting of coking said liquors by rapidly heating said liquors to the coking temperature at the rate of about 1 l0 to about F. per minute in the temperature interval of 350 to about 550F. whereby coke gel formation is avoided and coke separation is improved in the coked pulping liquors.

2. The process of claim 1 wherein said rapid heating is effected by the use of a liquid metal heat transfer agent.

3. The process according to claim 1 wherein said rapid heating is effected by direct heating with superheated steam.

4. The process according to claim 1 in which ammonia base sulfite pulping liquor is reconstituted by dissolving said sulfur dioxide with ammonia in said effluent.

5. The process of claim 1 wherein said rapid heating is effected by means of a fluidized bed combustion system.

6. The process to claim 5, additionally characterized by the steps of separating said sulfur dioxide from said coke and said effluent; burning said coke to provide process heat and a flue gas containing carbon monoxide; shifting said carbon monoxide to carbon dioxide and hydrogen; converting said hydrogen to ammonia with nitrogen and reconstituting pulping liquor by contacting said effluent with said sulfur dioxide and said ammonia.

7. The process of claim 6 wherein said carbon dioxide is recycled to said fluidized bed combustion system for temperature control and to adjust the CO concentration in said flue gas and hence the hydrogen concentration in said shifted flue gas.

8. The process of claim 6, wherein said converting to ammonia takes place at a temperature of around 750F. and under a pressure not exceeding 1500 psig.

=l l l 

1. IN A PROCESS FOR COKING SPENT AMMONIA AND LOW PH SODIUM BASE SULFITE WOOD PULPING LIQUORS WHEREIN THE LIQUOR ARE COKED ON THE LIQUID PHASE UNDER PRESSURE IN COKING ZONE AT A TEMPERATURE IN THE RANGE OF 400* TO 700*F. TO PRODUCE COKE, GASES INCLUDING SULFUR DIOXIDE, AND AN EFFLUENT, THE IMPROVMENT CONSISTING OF COKING SAID LIQUORS BY RAPIDLY HEATING SAID LIQUORS TO THE COKING TEMPERATURE AT THE RATE OF ABOUT 110* TO ABOUT 150*F. PER MINUTE IN THE TEMPERATURE INTERVAL OF 350* TO ABOUT 550*F. WHEREBY COKE GEL FORMATION IS AVOIDED AND COKE SEPARATION IS IMPROVED IN THE COKED PULPING LIQUORS
 2. The process of claim 1 wherein said rapid heating is effected by the use of a liquid metal heat transfer agent.
 3. The process according to claim 1 wherein said rapid heating is effected by direct heating with superheated steam.
 4. The process according to claim 1 in which ammonia base sulfite pulping liquor is reconstituted by dissolving said sulfur dioxide with ammonia in said effluent.
 5. The process of claim 1 wherein said rapid heating is effected by means of a fluidized bed combustion system.
 6. The process to claim 5, additionally characterized by the steps of separating said sulfur dioxide from said coke and said effluent; burning said coke to provide process heat and a flue gas containing carbon monoxide; shifting said carbon monoxide to carbon dioxide and hydrogen; converting said hydrogen to ammonia with nitrogen and reconstituting pulping liquor by contacting said effluent with said sulfur dioxide and said ammonia.
 7. The process of claim 6 wherein said carbon dioxide is recycled to said fluidized bed combustion system for temperature control and to adjust the CO concentration in said flue gas and hence the hydrogen concentration in said shifted flue gas.
 8. The process of claim 6, wherein said converting to ammonia takes place at a temperature of around 750*F. and under a pressure not exceeding 1500 psig. 